Current television transmission standards in Europe and other parts of the world operate with a 50 Hz display refresh rate. One complete image, or frame, is transmitted every 1/25th of a second. Each frame is composed of two half-images, or fields, each one of which is transmitted every 1/50th of a second. Each field consists of either the odd or even lines of a frame. The process of refreshing a CRT display causes a change of light intensity with time, called flicker. At 50 Hz, the human eye is still sensitive to this flicker, and on bright scenes and/or large displays this flicker is noticeable. One method of removing the visibility of this flicker is to refresh the display at a higher frequency. This increase in field rate may be by any arbitrary amount as long as a frequency is chosen which is sufficiently great that the sensitivity of the eye is very low. The frequency used as an example in this description is 100 Hz, a doubling in frequency which results in a relatively simple hardware implementation, although in principle, any frequency may be used and 75 Hz has also been comtemplated.
With the example frequency of 100 Hz, there are two scanning patterns which may be used, these are known as AABB and ABAB, where the letters A and B refer to the positions of the odd and even fields F. The arrangements of the two types of scanning is shown in FIGS. 1A and 1C. Lines L which belong to the original input signal are marked by a `o` and new lines which must be created are marked by a `x`.
The data in the new fields may be generated from the original input signal as illustrated by the arrows in FIG. 1. This presents an acceptable picture quality when the images are static. When the images move, the subjective effect in each case is one of a double image. This is due to the data in the new fields being displayed in the wrong temporal position, and when the eye tracks the motion across the screen, a double image of one form or another results. This effect is shown in FIGS. 1B and 1D, the latter of which shows `mouth teeth` artifacts.
This appearance of a double image around moving objects may be reduced or removed by generating the new 100 Hz fields with the object in the correct position. This is done by measuring the velocity with which the object is moving and then interpolating the new field using information from the correct position (i.e., from the object) in the source fields. This technique is called motion compensated interpolation, and in this instance is applied to frame/field rate upconversion, but could equally well have other uses, for example in the generation of an improved slow motion video signal, or in standards conversion.
One technique of motion estimation is known as block matching. Typically, the current field or frame of the picture is divided into rectangular blocks. For each block, a search is made over overlapping blocks in the previous field or frame to find the one that matches the current field block the best according to some criterion, typically mean-square or mean absolute error. The relative position of the two blocks gives the displacement, or motion, vector for the current block.